In a study of thyroid disease published in 1995, smokers had a higher survival rate (76 percent) over twenty years than nonsmokers (69 percent). However, the nonsmokers had a better survival rate in six out of seven age groups, and the difference was minimal in the seventh. Age was clearly a confounder of Smoking and Survival: the average smoker was younger than the average nonsmoker (perhaps because the older smokers had already died). Stratifying the data by age, we conclude that smoking has a negative impact on survival.
